The present invention pertains to well casings used in well drilling applications, and in particular to an integral well filter and screen used as a casing in horizontal wells, and methods for making and using same.
Wells generally are either vertical or horizontal. Vertical wells are more commonly known, but in certain applications horizontal wells have several advantages. Environmental site remediation, i.e., the removal of subsurface contamination or inplace treatment of zones of soil or groundwater contamination, is one area where horizontal wells are superior to vertical ones. Reasons for the superiority of horizontal wells include increased linear footage of the well in contact with the contaminated zone, and the ability to drill beneath surface obstructions or existing sites without the disturbing ongoing operations at the site. A horizontal well may be either single-ended or double-ended. In a single-ended well, one end of the well is at the ground surface and the other is below ground; this is also known as a xe2x80x9cblind hole.xe2x80x9d In a double-ended completion, both ends of the well are at the ground surface.
Completing a horizontal well usually involves two phases: drilling a well bore and installing a casing. Drilling begins with digging a launch pit at the point where the drill head will be inserted into the ground and, if it is a double-ended well, digging an exit pit where the drill head will emerge from the ground. The drill head has a transmitter therein which broadcasts a signal that enables the operator to determine its exact location, and is controlled and steered either remotely through radio signals from a control unit or directly through input provided through wires inside the drill, rods. The drill head is inserted into the ground at the launch pit and drilling begins. The operator guides the drill head downward to the desired depth, horizontally for the desired length of the well, and, if it is a double-ended well, upward to the exit pit. Different and more complex well structures, for example with several horizontal portions at different depths and orientations, are also possible.
Installing the casing is the second part of the well completion. The casing is a generally tubular member whose functions include keeping the well bore from caving in and transporting contaminants to the surface once the well is operating. The casing usually consists of a perforated well screen through which liquids and gases can enter and leave the well screen, and may include a means of filtration coupled thereto. The installation method will depend on whether the well is single-ended or double-ended. In a double-ended well, the drill head pulls behind it a drill string while it is guided from the launch pit to the exit pit; when the drill head emerges at the exit pit, the drill string emerges with it. A well casing is attached to drill string and the drill string is pulled from the exit pit back toward the launch pit, pulling the casing with it back through the well bore. In a single-ended well, the drill head is removed from the well bore when the drilling is complete and the casing is then pushed into the well bore through the launch pit. In either case, movement of the casing through the well bore may be facilitated using a lubricant such as drilling mud or guar-gum fluid. If a well screen is fragile, it can be installed using a carrier casing. After the well bore is drilled, it is reamed out and a solid casing with an inner diameter larger than the outside diameter of the well screen is installed. The well screen is placed within and pulled through the carrier casing. The carrier casing is then removed, leaving the well screen in place. Carrier casings make the installation expensive.
Despite their advantages, horizontal wells present special installation and operation problems that vertical wells do not. Horizontal well casings are subject to higher stresses and have a higher potential for damage during installation than vertical wells of similar dimensions. Stresses in a vertical casing tend to be low and only become a problem if the wells are very deep, so that gravitational forces acting on the casing are large. By contrast, horizontal casings are subjected to higher tensile forces because of frictional forces between the casing and the well bore and because of stress concentration in the casing as it bends to conform to turns in the well bore. Frictional forces on the casing are more pronounced in horizontal wells because the weight of the casing pulls it into contact with the lower part of the well bore. Stress concentrations are higher in horizontal casings because more force must be applied to overcome the friction and because of bending stresses induced in the casing when it is forced to conform to turns in the well bore that are characteristic of horizontal wells. Furthermore, horizontal well casings may be required to resist crushing forces that result if the well bore collapses onto the casing. In addition to experiencing higher stresses, horizontal casings are also more likely to receive abrasion damage from the sides of the well bore as the casings are pushed or pulled through the well bore.
Aside from installation problems, the most pronounced operational difference between vertical and horizontal wells is the increased tendency of soil, sand, and other fine particulate matter to enter the well casing through perforations in the well screen. Over long periods of time, particulate matter in the casing can lead to clogging of the well, damage to pumping equipment connected to the casing at the surface, and a variety of other problems. The usual way of dealing with this problem in vertical wells is to apply a filter pack to the casing. The orientation of the well casing in a horizontal well makes application of a filter pack more difficult.
In a vertical well, the well casing is held at or near the center of the well bore using a type of spacer known as a centralizer. Loose sand is distributed around the well casing to form a natural filter pack, with the sand gradation and slot size of the screen being chosen to provide the appropriate filtering. Natural filter packs have been unsuccessful in horizontal wells because it is difficult to create a sand pack completely surrounding the casing, numerous centralizers must be used to support the weight of the horizontal casing; the centralizers cause drag and high forces on the casing during installation.
Three main types of filter packs have been tried on horizontal wells with varying degrees of success. The simplest is the xe2x80x9cnatural pack,xe2x80x9d which essentially is a well screen with no filter at all. The natural pack can work quite well, depending on the use of the well and the grain size of the soil. If, however, the application is one that is prone to sedimentation, such as a groundwater extraction well, the natural pack does not work as well. Or, if it is used in an injection well, it can become silted or sanded if the operation of the well is cycled or temporarily suspended.
The next type of filter pack used on horizontal wells is the field constructed filter, also known as a xe2x80x9cfield wrap.xe2x80x9d The usual way of doing this is to wrap some form of non-woven filter material around the outside of the well screen. This method provides extra filtration, but can be expensive and time consuming, especially in the field. The filter material is robust, but it tends to increase the frictional drag between the well bore and the casing during installation. If a snag occurs during installation, the casing may break or the filtration material may become dislodged, leaving gaps through which soil can enter the screen.
The third type of filter pack is the integrated filter pack, and these come in two varieties. The first uses a synthetic well screen with wide slots and is covered with an external, tubular composite of filtration materials. The composite consists of a layer of fine, medical-grade synthetic mesh sandwiched between two layers of heavier mesh, and is installed on the well screen with heat shrink tubing bonded to the ends. Installation of this type of filter pack in a horizontal well bore is difficult due to the poor bonding of the filter to the base pipe, and therefore usually requires a carrier casing for installation. The second variety of integrated filter pack consists of a porous filter screen made of sintered polyethylene resin beads. The porous construction of the filter screen, however, limits its tensile strength and makes a carrier casing a requirement for installation.
The present invention is a well casing comprising an integral well filter and screen for use in well installations and a method for making and using the casing. One embodiment includes a well screen with a filter placed in an internal volume of the screen. The well screen is typically made of high-density polyethylene and has therein a plurality of perforations to allow fluid communication between the inside of the well screen and the outside. The well screen can be manufactured to any length. The filter includes a pliable filter material to which is bonded a substantially rigid but deformable filter support that keeps the filter material in contact with an inner surface of the well screen. The first embodiment is manufactured by bonding the filter support to the filter material, forming the filter assembly into a shape that fits into the internal volume and keeps the filter material in engagement with an inner surface of the well screen, inserting the filter assembly into the internal volume, and securing the assembly therein.
A second embodiment also has a well screen with an internal filter, but in this embodiment the filter is made of a substantially rigid material so that it needs no filter support. Instead, the filter material is simply formed into a cylindrical shape corresponding to the interior dimension of the well screen, joined at the abutting edges, and inserted inside the well screen as is the first embodiment.
The embodiments of the invention solve several of the problems of the prior art. Because the filter is inside the well screen, filter damage during insertion of the screen into the well bore is eliminated or substantially reduced. Putting the filter inside the screen and using a pliable filter material with a filter support bonded thereto result in a thinner cross section that allows the screen to bend easily when it goes around corners in well bores. The configuration also allows better tailoring of the stiffness of the combination to the application, has excellent hydraulic performance, is more durable, and costs half as much as present screen/filter casings.